Zamazenta is 10 faster with no shield, in trade for 30 Def and SpDef.

It also trades its fighting, ground, and fire weaknesses (very common types) to Flying, Psychic, Fairy, types it can handle much better through coverage moves like Crunch, Behemoth Bash, and Wild Charge.

Now, it does lose all those nice steel resistances and the poison immunity. But I submit to you that an Assault Vest, or even Banded, Scarfed, or Leftovers Zamazenta is a markedly better pokemon for competitive than its fancy special form (and especially for raiding, all ya'll dumbasses cut that out). Not to mention, it doesn't look as derpy, just as a little cherry on top.